Enterococci are leading causes of nosocomial infections, which affect approximately 2.5 million patients in the US each year. Nosocomial infections in general, and enterococcal infections in particular, are frequently refractory to antibiotic treatment because of multiple antibiotic resistances. Few studies have examined the pathogenesis of enterococcal infection. One of the factors that has been thoroughly documented to contribute to the virulence of Enterococcus faecalis is the cytolysin. The cytolysin is a structurally novel toxin that consists of two dissimilar, small subunits. Each subunit is extensively post-translationally modified and proteolytically processed during maturation. Mature subunits interact with target cell membranes to effect lysis of eukaryotic cells, and killing of bacterial cells of gram-positive species. We recently described a novel, two-component regulatory system that governs expression of the E. faecalis cytolysin. The two regulatory components function by an unknown mechanism to shut off transcription of the cytolysin operon. Interestingly, the operon is induced by the smaller of the two toxin subunits, but only in its mature and fully processed form. Thus, the small toxin subunit must be translated, post-translationally modified, secreted, and processed through two proteolytic trimming steps before it can feed back onto the system and induce high level expression of the cytolysin operon. As the cytolysin contributes to the toxicity of enterococcal infection, it is of interest to decipher the molecular mechanisms involved in toxin regulation, and in toxin activity, toward the goal of developing a therapeutic that renders cytolytic enterococcal infections less destructive. The aims of this proposal therefore are to determine the molecular mechanism by which the two components CylR1 and CylR2 regulate expression of the cytolysin operon; and to determine how the structures of the toxin subunits CylLL and CylLS, including post-translational modifications, relate to cytolytic bactericidal and cytolysin operon inducing function.